The present invention pertains to a manually operated, engine driven vibratory concrete screed and, more particularly, to an improved screed bar for such a screed.
Vibratory screeds are used to smooth the surface of freshly poured concrete and to eliminate air pockets within the concrete mass. One type of manually operated screed is driven by a small gasoline engine (e.g. 1 to 2.5 horsepower) that turns an eccentric exciter mechanism to impart a high speed vibratory force to a screed bar attached to the exciter mechanism. This type of vibratory screed includes an operating handle connected through a frame to the vibratory exciter and engine. The machine is pulled over the surface of the concrete and a small amount of fresh concrete will build up behind the bar to ensure that the surface is uniform and depressions are not created. The blade may be up to 24 feet in length, and, although vibration of the blade helps make the concrete flow, the operator must still pull the machine over the surface of the concrete.
Many types of screed bars have bee n used, arranging from relatively crude, solid wooden bars of rectangular cross-section to more sophisticated, hollow metal extrusions. In particular, long metal extrusions, typically made of magnesium or aluminum, have been made with a variety of cross-sections, including rectangular, triangular, trapezoidal and other various shapes.
It has been found, however, that in actual use, particularly as the length of the screed bar increases, the vibration is not uniformly distributed along the entire length of the bar and, as a result, the finishing of the surface of the concrete is not uniform. This lack of uniform distribution of vibration over the full length of the screed bar and the consequent non-uniform finishing of the concrete surface has been shown to occur in testing all of the various prior art screed bars. In addition, screed bar strength becomes increasingly important as the length of the screed bar increases. Even with the use of lightweight metal extrusions, the screed bar must have adequate stiffness to prevent excessive bending at longer screed bar lengths.
There remains a need, therefore, for a lightweight screed bar of adequate strength over the full length of which the vibrations may be more uniformly distributed so that the concrete surface is uniformly finished.